Solar cells and solar modules convert sunlight into electricity. These electronic devices have been traditionally fabricated using silicon (Si) as a light-absorbing, semiconducting material in a relatively expensive production process. To make solar cells more economically viable, solar cell device architectures have been developed that can inexpensively make use of thin-film, light-absorbing semiconductor materials such as copper-indium-gallium-sulfo-di-selenide, Cu(In,Ga)(S,Se)2, also termed CI(G)S(S). This class of solar cells typically has a p-type absorber layer sandwiched between a back electrode layer and an n-type junction partner layer. The back electrode layer is often Mo, while the junction partner is often CdS. A transparent conductive oxide (TCO) such as zinc oxide (ZnOx) doped with aluminum is formed on the junction partner layer and is typically used as a transparent electrode. CIS-based solar cells have been demonstrated to have power conversion efficiencies exceeding 19%.
A central challenge in cost-effectively constructing a large-area copper-indium-gallium-di-selenide (CIGS) based solar cell or module is that the elements of the CIGS layer must be within a narrow stoichiometric ratio on nano-, meso-, and macroscopic length scale in all three dimensions in order for the resulting cell or module to be highly efficient. Achieving precise stoichiometric composition over relatively large substrate areas is, however, difficult using traditional vacuum-based deposition processes. For example, it is difficult to deposit compounds and/or alloys containing more than one element by sputtering or evaporation. Both techniques rely on deposition approaches that are limited to line-of-sight and limited-area sources, tending to result in poor surface coverage. Line-of-sight trajectories and limited-area sources can result in non-uniform three-dimensional distribution of the elements in all three dimensions and/or poor film-thickness uniformity over large areas. These non-uniformities can occur over the nano-, meso-, and/or macroscopic scales. Such non-uniformity also alters the local stoichiometric ratios of the absorber layer, decreasing the potential power conversion efficiency of the complete cell or module.
Alternatives to traditional vacuum-based deposition techniques have been developed. In particular, production of solar cells on flexible substrates using non-vacuum, semiconductor printing technologies provides a highly cost-efficient alternative to conventional vacuum-deposited solar cells. For example, T. Arita and coworkers [20th IEEE PV Specialists Conference, 1988, page 1650] described a non-vacuum, screen printing technique that involved mixing and milling pure Cu, In and Se powders in the compositional ratio of 1:1:2 and forming a screen printable paste, screen printing the paste on a substrate, and sintering this film to form the compound layer. They reported that although they had started with elemental Cu, In and Se powders, after the milling step the paste contained the Cu—In—Se2 phase. However, solar cells fabricated from the sintered layers had very low efficiencies because the structural and electronic quality of these absorbers was poor.
Screen-printed Cu—In—Se2 deposited in a thin-film was also reported by A. Vervaet et al. [9th European Communities PV Solar Energy Conference, 1989, page 480], where a micron-sized Cu—In—Se2 powder was used along with micron-sized Se powder to prepare a screen printable paste. Layers formed by non-vacuum, screen printing were sintered at high temperature. A difficulty in this approach was finding an appropriate fluxing agent for dense Cu—In—Se2 film formation. Even though solar cells made in this manner had poor conversion efficiencies, the use of printing and other non-vacuum techniques to create solar cells remains promising.
It should be understood that some precursor materials used in non-vacuum manufacturing of thin-films suitable for semiconductor devices may be in liquid form, with these precursor materials serving as source material for the thin-film, whereas most other precursor materials in the ink are in solid form and desirably so, this in contrast to materials added to the ink to allow for reliable, fast and uniform deposition, like solvents and organic additives. These solvents and organic additives are typically unwanted in the final thin-film and require facile removal during or after the deposition process. Unfortunately, sometimes these preferably solid components can become liquid at the handling and/or particle size reduction temperatures typically associated with non-vacuum techniques for solar cell production. This may be a disadvantageous feature as premature and/or undesired liquification or coalescence increases the difficulty in handling these materials during processing, during ink storage, and may require more involved techniques. For example, elemental gallium is a liquid above 30° C., which is very close to room temperature and below the processing temperature associated with deposition and/or ink preparation. It may also be disadvantageous during processing since the liquid form may change the kinetics of the conversion of the particulate layer to the final semiconductor film. For example, if too much liquid is present at or near the onset of a reaction, liquid may group together at certain areas and not be evenly distributed throughout the reaction area. This can result in thickness non-uniformity and/or lateral composition non-uniformity. Furthermore, if material in liquid form leaches out from an alloy or compound containing that material, this may change the local stoichiometry at the start of the reaction resulting in different compound(s) in the final thin-film if the leaching occurs prior to or during processing of the materials.
For example for the preferably solid components, liquid form might be present and undesirable before/during the synthesis of the particles. Such components in liquid form increases the difficulty in controlling and maintaining the particle (droplet) size during ink preparation and solution deposition. In one example, elemental gallium used in thin-film solar cell production is a liquid above 30° C., which is very close to room temperature and below the processing temperature typically used during ink deposition. Lowering the processing temperature far below the melting point of gallium complicates the ink preparation and solution deposition. Additionally, difficulty in controlling the particle (droplet) size during deposition complicates controlling and maintaining the target thickness uniformity of the resulting film on micro-, and macroscopic length scales.
Additionally for the preferably solid components, liquid might be present and undesirable when annealing the coatings of ink. It may also be disadvantageous during single and/or multi-step conversion of the solution-deposited coating or layer into the resulting semiconductor film since the premature presence of liquid may change the kinetics of the reactions involved and therefore the quality and uniformity of the semiconductor film. For example, if too much liquid is present at or near the onset of a reaction, liquid may dewet from the surface and ball up resulting in a non-uniform material distribution throughout the layer, both in thickness and composition.
Due to the aforementioned issues, there are significant opportunities for improving non-vacuum CIGS manufacturing processes. Improvements may be made to increase the throughput of existing CIGS manufacturing process and decrease the cost associated with CIGS based solar devices. The decreased cost and increased production throughput should increase market penetration and commercial adoption of such products.